I use sugar, Yeast (like fleishmanns or something just basic yeast you can find in grocery stores) and water. for equipment i used a 2 liter bottle and another smaller bottle filled with only water. In teh 2liter bottle i dissolve about 2 cups of sugar into it. Yeast i use probably like 1/2->1 tsp yeast. Proof it for 20 minutes but mixing it with lukewarm water. u should see stuff happening. Then mix the two solutions. I leave some space between the water level and the top of the 2L bottle so you dont get crap flowing into the other bottle. From the 2L bottle, take a tube and route it into the smaller bottle filled with water. That simple acts as a water filter so the CO2 from the 2L bubbles through the water and up. Then from the small bottle, taht rube coming out goes to the diffuser. It will take about a day for you to see good CO2 production. I highly doubt you will ever produce enough co2 this method to gas your fish so don't really need to worry about too high levels. Im sure its possible, but very unlikely.

If you can find it, i mix my yeast with some wine yeast as well. Wine yeast can take higher alcohol levels but produces co2 at a lower rate. However, its very consistent. I tried all wine yeast before. It sucks. Almost no CO2 came out and it takes like 2 days to get going. However, I mixed the two and it wasnt that bad. Also wine yeast settles to the bottom whereas bakers yeast floats. When you change out the yeast solutions, leave some of the Original liquid in as it can help speed up fermentation process IME.

What diffuser are you using? The diffuser might be clogged if it still cant get out.